Paradoxical Downregulation of CXC Chemokine Receptor 4 Induced

Apr 10, 2012 - CXCR4 and its ligand stromal cell derived factor-1 (SDF-1)/CXCL12 play pivotal parts in many physiological processes and pathogenetic ...
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Paradoxical Downregulation of CXC Chemokine Receptor 4 Induced by Polyphemusin II-Derived Antagonists Ryo Masuda,† Shinya Oishi,*,† Noriko Tanahara,† Hiroaki Ohno,† Akira Hirasawa,† Gozoh Tsujimoto,† Yoshiaki Yano,† Katsumi Matsuzaki,† Jean-Marc Navenot,‡ Stephen C. Peiper,‡ and Nobutaka Fujii*,† †

Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Department of Pathology, Anatomy and Cell Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, United States

‡

S Supporting Information *

ABSTRACT: CXC chemokine receptor 4 (CXCR4) is a G protein-coupled receptor implicated in cell entry of T-cell linetropic HIV-1 strains. CXCR4 and its ligand stromal cell derived factor-1 (SDF-1)/CXCL12 play pivotal parts in many physiological processes and pathogenetic conditions (e.g., immune cell-homing and cancer metastasis). We previously developed the potent CXCR4 antagonist T140 from structure−activity relationship studies of the antimicrobial peptide polyphemusin II. T140 and its derivatives have been exploited in biological and biomedical studies for the SDF-1/ CXCR4 axis. We investigated receptor localization upon ligand stimulation using fluorescent SDF-1 and T140 derivatives as well as a specific labeling technique for cellular-membrane CXCR4. Fluorescent T140 derivatives induced translocation of CXCR4 into the perinuclear region as observed by treatment with fluorescent SDF-1. T140 derivative-mediated internalization of CXCR4 was also monitored by the coiled-coil tag-probe system. These findings demonstrated that the CXCR4 antagonistic activity and anti-HIV activity of T140 derivatives were derived (at least in part) from antagonist-mediated receptor internalization.

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pathway or for recycling endosomes.11,14 In addition, CXCR4 is used as a major co-receptor for the entry of T-cell line tropic human immunodeficiency virus type 1 (HIV-1) into target host cells.15,16 The inhibitory effect of SDF-1 on HIV infection is thought to be by competitive binding to CXCR4 as well as CXCR4 downregulation.17,18 CXCR4 is a promising molecular target for potential anti-metastatic agents and anti-HIV agents, so several CXCR4 ligands have been developed.5,19−22 We previously developed the potent anti-HIV peptide T140. This was designed from the structure−activity relationship studies of a self-defense peptide of horseshoe crabs, polyphemusin II (Figure 1).19 Inhibition of HIV-induced cytopathogenicity by T140 and its derivatives was derived from selective CXCR4 antagonistic and/or inverse agonistic activity (in which the basal signal levels were decreased in the guanosine triphosphate (GTP) binding and intracellular calcium flux assay using a constitutively active mutant).23 A recent report on the crystal structure of CXCR4 in complex with the T140 derivative CVX15 revealed that arginine residues in CVX15 made polar interactions with Asp171 and Asp187 in CXCR4.24 Point-mutation experiments of CXCR4 revealed that additional residues on the extracellular domain (Arg188, Gly207, and Asp262) are necessary for the interaction of

INTRODUCTION CXC chemokine receptor 4 (CXCR4) is a G-protein-coupled receptor. It is widely expressed in leukocytes such as T-cells, Bcells, and monocytes.1,2 Under physiological conditions, the endogenous ligand, stromal cell-derived factor-1 (SDF-1)/ CXCL12, is secreted by bone marrow stromal cells for expansion and development of precursor B-cells.3 High concentrations of SDF-1 are present at inflammatory sites, so the migration of CXCR4-expressing stem cells toward an SDF1 gradient promotes repair of injured tissues.4 There have been many reports on the pathology of CXCR4-related cancer, including CXCR4 overexpression and organ-specific metastasis among various types of cancer cells.5,6 During metastasis, SDF1 from secondary lesions functions as a chemoattractant for directional migration of CXCR4-expressing malignant cells.5,6 The activation process of CXCR4 by SDF-1 has been well documented.7 Upon SDF-1 binding, CXCR4 evokes downstream signaling via dissociation of heterotrimeric G proteins, followed by decrease in intracellular cyclic adenosine monophosphate (cAMP) concentrations, upregulation of Ca2+ release, and increase in extracellular-signal-regulated kinase (ERK) 1/2 phosphorylations.8−10 Furthermore, CXCR4 internalization in response to SDF-1 occurs in early endosomes through β-arrestin recruitment, just like other GPCRs,11 in which phosphorylated serine residues and a dileucine motif at the CXCR4 C-terminus have critical roles.12,13 The complex is sorted into late endosomes/lysosomes for the degradation © 2012 American Chemical Society

Received: February 18, 2012 Revised: April 7, 2012 Published: April 10, 2012 1259

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Figure 1. Amino acid sequences of CXCR4 antagonist peptides. The disulfide bonds between cysteine residues are shown by solid lines. D-Amino acids are not first-letter capitalized. Abbreviations: Nal, L-3-(2-naphthyl)alanine; Cit, L-citrulline; 4FBz, 4-fluorobenzoyl; AF488, AlexaFluor 488; TMR, tetramethylrhodamine.

T140, which is independent of the interaction with SDF-1.25,26 These residues contribute to stabilization of the CXCR4 structure via formation of hydrogen bonds with adjacent residues, so T140 binding to these residues may impair SDF-1 binding via conformational changes of CXCR4. ALX-40C is an alternative CXCR4 antagonist that inhibits HIV-1 infection into T-cells.27 The anti-HIV activity of ALX-40C with a sequence of nine D-Arg is also reported to be derived from binding to acidic residues in the second extracellular loop of CXCR4.27 It was demonstrated that highly basic peptides such as ALX40C and HIV-1 Tat translocate into the intracellular compartment through endocytosis.28−31 On the basis of the biological properties of these highly basic peptides, we assumed that polyphemusin II-derived CXCR4 antagonists could induce the internalization and/or translocation of receptors into the intracellular compartment, resulting in the apparent antagonistic or inverse agonistic activity for CXCR4. In the present study, we investigated the mechanism of action of T140 derivatives using novel fluorescent probes. The uptake and localization of CXCR4 was monitored by analyses of flow cytometry and confocal microscopy using the coiled-coil tag-probe system32 for the labeling of cell-surface CXCR4.

Waltham, MA, USA) pretreated with 0.1% polyethyleneimine. The filter plate was washed with wash buffer [50 mM HEPES (pH 7.4), 500 mM NaCl, and 0.1% BSA in H2O] and bound radioactivity measured by TopCount (PerkinElmer). Confocal Microscopy Analyses of Ligand and CXCR4 Internalization. E3-CXCR4 CHO cells were plated on 35 mm glass-bottomed dishes and cultured in F-12 medium containing 10% heat-inactivated fetal bovine serum supplemented with penicillin/streptomycin and hygromycin. Cells were washed once with cold F-12 medium, and incubated with fluorescent ligands [SDF-1AF488 (100 nM) or TY14015 (1 μM)] in F-12 medium (100 μL) at 30 °C for 30 min. After rinsing once with cold F-12 medium, cells were observed by confocal microscopy (Eclipse Ti-E: Nikon, Tokyo, Japan). To monitor CXCR4 localization, E3-CXCR4 CHO cells were pretreated with 100 nM fluorescent K4-peptide in F-12 medium (100 μL) at 0 °C for 15 min before treatment with CXCR4 ligands. For staining of cellular compartments, after incubation with fluorescent ligands or K4-peptide, cells were rinsed once with cold F-12 medium and treated with the marker (FM 4−64 for the cell membrane; LysoTracker Red DND-99 for lysosomes; ER-tracker Red for the endoplasmic reticulum; or AlexaFluor 568-conjugated transferrin for endosomes) according to the manufacturer (Invitrogen, Carlsbad, CA, USA, for all markers) protocol. The green (AlexaFluor 488 and ATTO488) channel was excited by a 488 nm laser and detected through a BP 500− 550 nm emission filter. The red (TMR and AlexaFluor 568) channel was excited by a 568 nm laser, and detected through a BP 575−605 nm emission filter. The blue (FM 4−64) channel was excited by a 568 nm laser, and detected through a LP 665 nm emission filter. Data were analyzed using EZ-C1 Viewer software (Nikon).

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EXPERIMENTAL SECTION Quantitative Analyses of Cell-Surface E3-CXCR4 Using a Fluorescent K4-Peptide by Flow Cytometry. E3-CXCR4 CHO cells were detached using versene and incubated with 100 nM FL-K4 in F-12 medium (500 μL) at 0 °C for 15 min in the absence or presence of unlabeled K4-peptide or SDF-1. The intensity of cell staining was analyzed using a FACScalibur system (BD Biosciences, San Jose, CA, USA). Ten thousand events per sample were analyzed, and the data collected from FL1 in log mode. Fluorescent intensity was calculated as a geometric mean of cellular fluorescence. Data were analyzed using CellQuest Pro software (BD Biosciences). Data were analyzed using a two-tailed Student’s t test with significance set at p ≤ 0.05. Quantitative Analyses of Ligand-Mediated CXCR4 Internalization by Flow Cytometry. E3-CXCR4 CHO cells were detached using versene and resuspended in F-12 medium (100 μL) containing each ligand. After incubation at 37 °C for 30 min, ice-cold F-12 medium (400 μL) was added to the mixture, and cells centrifuged at 500 × g for 5 min at 4 °C. Cell pellets were then incubated with 100 nM FL-K4 in F-12 (100 μL) at 0 °C for 15 min. The mixture was diluted with icecold F-12 medium (400 μL), and analyzed using a FACScalibur flow cytometer. Binding and Displacement of [125I]-SDF-1. A membrane fraction of cells expressing CXCR4 was incubated with 0.5 nM of [125I]-SDF-1 and FL-K4 in binding buffer [50 mM HEPES (pH 7.4), 5 mM MgCl2, 1 mM CaCl2, and 0.1% bovine serum albumin (BSA) in H2O] for 1 h at room temperature. Reaction mixtures were filtered through GF/B filters (PerkinElmer,

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RESULTS Labeling of Cell-Membrane CXCR4 by the Coiled-Coil Tag-Probe System. A stable CXCR4-expressing cell line was established to monitor the internalization of CXCR4. The surface-exposed tag sequence E3 (EIAALEK)3 was appended at the N-terminus of CXCR4 for detection using the peptide probe K4 (KIAALKE)4 with an appropriate tracer group.32 This coiled-coil tag-probe system provides several distinct advantages to visualize cell-surface CXCR4. For example, K4-peptide is much smaller than anti-epitope antibodies, so ligand binding to the receptor is hardly disturbed. In addition, specific labeling of E3-tagged receptors on cell membranes with fluorescent K4peptides can distinguish the internalized receptor from the receptor that is originally present in the cytosolic compartment. This is in contrast to receptors fused with fluorescent proteins, which have usually been employed for monitoring receptor localization.14,33 CHO cells stably expressing E3-tagged CXCR4 (E3CXCR4) were generated by the Flp-In expression system, and were studied by flow cytometric analyses. E3-CXCR4 1260

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Figure 2. Labeling of E3-CXCR4 cells with fluorescein-K4-peptide (FL-K4). (a) FL-K4 binding to CXCR4 cell lines. After cells were treated with FL-K4 (100 nM) at 0 °C for 15 min, bound FL-K4 was measured by flow cytometry. (b) Inhibition of FL-K4 binding to E3CXCR4 by unlabeled K4-peptide. After E3-CXCR4 CHO cells were labeled with FL-K4 (100 nM) in the presence of various concentrations of unlabeled K4-peptide at 0 °C for 15 min, bound FL-K4 was measured by flow cytometry. (c) Effect of SDF-1 on FL-K4 binding to E3-CXCR4. After E3-CXCR4 CHO cells were labeled with FL-K4 (100 nM) in the presence of various concentrations of SDF-1 at 0 °C for 15 min, bound various concentrations of FL-K4 was measured by flow cytometry. (d) Effect of K4-peptide on SDF-1 binding to E3-CXCR4. [125I]-SDF-1 (0.5 nM) binding to E3-CXCR4. (±S.D., n = 3; *** p ≤ 0.005).

CHO cells were clearly seen to be stained by fluoresceinconjugated K4-peptide (FL-K4) (Figure 2a). This staining was inhibited by unlabeled K4-peptide in a dose-dependent manner, suggesting specific labeling of E3-CXCR4 by interaction between E3-tag and K4-peptide (Figure 2b). FL-K4 binding to E3-CXCR4 was not disturbed by SDF-1 even at 1 μM, which demonstrated that FL-K4-mediated staining was independent of SDF-1 binding to CXCR4 (Figure 2c). SDF-1 binding to E3CXCR4 was also unaffected by K4-peptide, which was verified by the binding inhibition assay using [125I]-SDF-1 (Figure 2d). Taken together, specific fluorescent labeling of CXCR4 was accomplished by the coiled-coil tag-probe system without mutual competitive inhibition of SDF-1 and K4-peptide binding to the receptor. Monitoring and Quantitative Analyses of SDF-1Induced CXCR4 Internalization. The level of residual CXCR4 on the cell membrane after SDF-1 stimulation has been measured by flow cytometry using a CXCR4-specific antibody.34,35 For example, Honczarenko et al. assessed SDF-1induced internalization of CXCR4 by staining cell-surface CXCR4 by the monoclonal antibody 12G5.34 However, the possible competitive binding of SDF-1 and antibody to cellsurface CXCR4 may impair receptor detection. To overcome this potential disadvantage, the coiled-coil tag-probe pair system could be an alternative to quantify cell-surface CXCR4.

Figure 3. CXCR4 internalization induced by SDF-1 derivatives. (a) Quantitative analyses of E3-CXCR4 internalization by flow cytometry. After E3-CXCR4 CHO cells were treated with various concentrations of SDF-1 at 37 °C for 30 min, cells were labeled by FL-K4 (100 nM) at 0 °C for 15 min (±S.D., n = 3; * p ≤ 0.05; *** p ≤ 0.005). (b) Confocal microscopy images of SDF-1-mediated CXCR4 internalization. After E3-CXCR4-expressing cells were labeled with ATTO488-K4 (100 nM) at 0 °C for 15 min, cells were treated with SDF-1 (100 nM) at 30 °C for 30 min. (c) Confocal microscopy images of internalized fluorescent SDF-1. E3-CXCR4-expressing cells were treated with SDF-1AF488 (100 nM) for 30 min. (d) Confocal microscopy images of internalized SDF-1 (green) and CXCR4 (red). After E3-CXCR4-expressing cells were labeled with TMR-K4 (100 nM) at 0 °C for 15 min, and cells were treated with SDF-1AF488 (100 nM) at 30 °C for 30 min. Representative z-slice confocal microscopy images are shown.

SDF-1-mediated CXCR4 internalization was investigated using the established system. Initially, quantitative analyses of residual cell-surface E3-CXCR4 after SDF-1 stimulation were undertaken by flow cytometry. Cells were stained with FL-K4 after E3-CXCR4 CHO cells were treated with SDF-1 over a range of concentrations. The fluorescent intensity of FL-K4 was significantly decreased in a dose-dependent manner (Figure 3a). Confocal microscopy studies using ATTO488-conjugated 1261

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K4-peptide (ATTO488-K4) confirmed the translocation of E3CXCR4 to the intracellular compartment by treatment with SDF-1, whereas ATTO488-K4/E3-CXCR4 remained on the cell surface without SDF-1 treatment (Figure 3b). Both results suggested that SDF-1 stimulation induced FL-K4 labeled E3CXCR4 internalization, and that the E3 sequence on the Nterminus of CXCR4 could work as a functional tag to detect receptor localization without disturbing receptor internalization. CXCR4 internalization was also monitored by confocal microscopy using a fluorescent SDF-1 derivative.36 Translocation of SDF-1AF488 into the intracellular compartment was observed by treatment of E3-CXCR4 cells (without labeling by fluorescent K4-peptide) (Figure 3c). This was consistent with the SDF-1-mediated internalization of FL-K4-labeled E3CXCR4 (Figure 3b). The intracellular localization of SDF1AF488 was not observed in CHO cells without CXCR4 expression (see Supporting Information, Supplementary Figure 1). Hence, it could be concluded that this translocation was mediated by the interaction with CXCR4. To confirm the colocalization of SDF-1 and CXCR4, the same experiment was conducted using E3-CXCR4-expressing cells labeled with TMR-conjugated K4-peptide (TMR-K4). Incubation of cells with SDF-1AF488 induced translocation of TMR-K4/E3-CXCR4 complexes into the intracellular compartment, which was verified by colocalization of SDF-1AF488 and TMR-K4/E3CXCR4 (Figure 3d). An identical phenotype was observed in an experiment using SDF-1TMR and ATTO488-K4/E3-CXCR4, indicating that the fluorophore functional groups on the ligand and receptor did not influence the translocation (see Supporting Information, Supplementary Figure 2). Polyphemusin-Derived CXCR4 Antagonists Induce Receptor Internalization. Next, we investigated receptor internalization by CXCR4 antagonists using the coiled-coil tagprobe system. E3-CXCR4 CHO cells were treated with polyphemusin II and CXCR4 antagonists (TF1401637 or FC13138) at 37 °C. The proportion of residual receptors on the cell membrane was subsequently determined by flow cytometry in the presence of FL-K4 (Figure 4a). The fluorescence intensity of FL-K4 was reduced by 20−25% by the antagonists.39 In contrast, a decrease in FL-K4 fluorescence was not observed in the same experiment at 0 °C, in which receptor internalization does not occur,40 suggesting that the E3-tag/K4-peptide interaction was not inhibited by the antagonists. Antagonist-induced internalization was also confirmed by fluorescence microscopy analyses of ATTO488-K4/ E3-CXCR4 CHO cells. TF14016 induced the translocation of ATTO488-K4/E3-CXCR4 into the perinuclear region, just like that seen in SDF-1 stimulation (Figure 4b). As such, it was demonstrated that CXCR4 antagonists partially induced receptor internalization. The fluorescent CXCR4 antagonist TY14015 was similarly accumulated in the cytosolic perinuclear domain in E3-CXCR4 CHO cells after 30 min incubation at 30 °C (Figure 4c). TY14015-mediated translocation was not observed in the same experiments using nontransfected CHO cells,41 nor by incubation of E3-CXCR4 CHO cells with TY14015 at 0 °C (see Supporting Information, Supplementary Figure 1). These findings indicated that CXCR4 serves as an essential receptor for the translocation of CXCR4 antagonists by an active pathway such as endocytosis. In contrast to the experiment using SDF-1AF488 (Figure 3c), staining of the cell membrane by TY14015 was also observed (Figure 4c).

Figure 4. CXCR4 internalization upon stimulation by CXCR4 antagonists. (a) Quantitative analyses of E3-CXCR4 internalization by flow cytometry. After E3-CXCR4 CHO cells were treated with each antagonist (1 μM) at 37 °C for 30 min, and cells were labeled by FLK4 (100 nM) at 0 °C for 15 min (±S.D., n = 3; * p ≤ 0.05; *** p ≤ 0.005). (b) Confocal microscopy images of TF14016-mediated CXCR4 internalization. After E3-CXCR4 CHO cells were labeled with ATTO488-K4 (100 nM) at 0 °C for 15 min, cells were treated with TF14016 (1 μM) at 30 °C for 30 min. (c) Confocal microscopy images of internalized TY14015. E3-CXCR4 CHO cells were treated with TY14015 (1 μM) at 30 °C for 30 min. (d) Confocal microscopy images of internalized TY14015 (green) and CXCR4 (red). After E3CXCR4 CHO cells were labeled with TMR-K4 (100 nM) at 0 °C for 15 min, cells were treated with TY14015 (1 μM) at 30 °C for 30 min. Representative z-slice confocal microscopy images are shown.

The localization of fluorescent T140 derivatives and CXCR4 was simultaneously monitored by confocal microscopy using the coiled-coil tag-probe system. After preincubation with TMR-K4, E3-CXCR4 CHO cells were stimulated by TY14015. Merged confocal microscopy images revealed that TY14015 colocalized with TMR-K4/E3-CXCR4 (Figure 4d). This colocalization was not affected by the fluorophores, which was verified by experiments using ATTO488-K4/E3-CXCR4 and TR14011 (Figure 1, see also Supporting Information, Supplementary Figure 2). CXCR4-Venus CHO cells (in which a fluorescent Venus protein was fused at the C-terminus of CXCR4) showed an identical phenotype upon stimulation with 1262

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Intracellular Translocation of CXCR4 Ligands and Receptor. The intracellular destination of CXCR4 ligand− receptor complexes after the binding of CXCR4 antagonists was investigated by confocal microscopy (Figure 5; see also Supporting Information, Supplementary Figure 3). After E3CXCR4 CHO cells were treated with CXCR4 ligands, cells were stained with several organelle-specific fluorescent markers. SDF-1AF488 translocated to endosomal compartments in 30 min, which were stained by AlexaFluor 568-conjugated transferrin (Figure 5a). This result was in agreement with another report on SDF-1-mediated CXCR4 internalization.17 Similarly, TY14015 accumulated in the same intracellular compartment (Figure 5b). Localization of ATTO-K4/E3CXCR4 after treatment with SDF-1 and TF14016 was also similar to the distribution of transferrin, indicating that internalized CXCR4 ligand−receptor complexes translocate into endosomal compartments (Figure 5c,d). Meanwhile, localization of the fluorescent CXCR4 ligands in lysosomes was partial (Figure 5e,f). Agonists and antagonists of CXCR4 may indirectly affect the distributions of lysosomes. Staining with FM 4−64 or ER-trackers suggested that SDF-1AF488 existed neither on the cell membrane nor in the endoplasmic reticulum, whereas partial staining with TY14015 on cell membranes was observed (see Supporting Information, Supplementary Figure 3).

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DISCUSSION Agonist-mediated internalization of GPCRs induces the transduction of downstream signaling and desensitization to regulate cell homeostasis. In contrast, reports on antagonistinduced receptor internalization (e.g., cholecystokinin A, 5HT2A, and neuropeptide Y1 receptors) are limited.42−44 This is the first report on the antagonist-mediated internalization of CXCR4. A series of polyphemusin II-derived and other CXCR4 antagonists contain basic functional groups such as arginine and lysine residues, which are involved in the interactions with the extracellular domain of CXCR4-bearing negative charges. In the present study, using the coiled-coil tag-probe system to visualize cell-surface CXCR4, CXCR4-mediated translocation of T140 derivatives into intracellular compartments was demonstrated. Although the internalization effect of surface CXCR4 by T140 derivatives was partial (only 25%, much less than the agonist SDF-1), this CXCR4 internalization supports the apparent antagonistic activity of T140 derivatives against SDF-1 binding to CXCR4 as well as the induction of inverse agonistic activity signaling.23 It was reported that treatment of CXCR4-expressing cells with HIV-1 gp120 peptide induces similar CXCR4 internalization without agonistic activity, which is closely related to HIV infection.14 Although T140 derivatives have been thought to be competitive inhibitors against gp120 binding to CXCR4,19 antagonist-mediated internalization of cell-surface CXCR4 could be an alternative mode of action for anti-HIV activity.17,18 Further investigation of the mechanisms of this paradoxical antagonist-mediated down-regulation of CXCR4 could facilitate development of novel anti-metastatic and anti-HIV agents.

Figure 5. Translocation of fluorescent CXCR4 ligands and the receptor. (a,b) Confocal microscopy images of SDF-1AF488 (a) or TY14015 (b) and endosome. After E3-CXCR4 CHO cells were treated with fluorescent ligands at 30 °C for 30 min, cells were stained with AlexaFluor 568-transferrin (50 μg/mL) for 60 min. (c,d) Confocal microscopy images of internalized CXCR4 and endosome after stimulation with SDF-1 (c) or TF14016 (d). After E3-CXCR4 CHO cells were labeled with ATTO488-K4 (100 nM) at 0 °C for 15 min, cells were treated with ligand at 30 °C for 30 min and stained with AlexaFluor 568-transferrin (50 μg/mL) for 60 min. (e,f) Confocal microscopy images of SDF-1AF488 (e) or TY14015 (f) and lysosome. After E3-CXCR4 CHO cells were treated with fluorescent ligands at 30 °C for 30 min, cells were stained with LysoTracker (1 μM) for 30 min. Representative z-slice confocal microscopy images are shown.

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ASSOCIATED CONTENT

* Supporting Information S

SDF-1TMR or TR14011 (see Supporting Information, Supplementary Figure 2). These data suggested that T140 derivatives translocated into the intracellular compartment with formation of ligand−receptor complexes.

Experimental procedures as well as confocal microscopy images of ligand and receptor internalization in the control experiments. This material is available free of charge via the Internet at http://pubs.acs.org. 1263

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of arrestins and identification of residues in the C-terminal tail that mediate receptor internalization. J. Biol. Chem. 274, 31076−31086. (14) Tarasova, N. I., Stauber, R. H., and Michejda, C. J. (1998) Spontaneous and ligand-induced trafficking of CXC-chemokine receptor 4. J. Biol. Chem. 273, 15883−15886. (15) Feng, Y., Broder, C. C., Kennedy, P. E., and Berger, E. A. (1996) HIV-1 entry cofactor: functional cDNA cloning of a seven-transmembrane, G protein-coupled receptor. Science 272, 872−877. (16) Berson, J. F., Long, D., Doranz, B. J., Rucker, J., Jirik, F. R., and Doms, R. W. (1996) A seven-transmembrane domain receptor involved in fusion and entry of T-cell-tropic human immunodeficiency virus type 1 strains. J. Virol. 70, 6288−6295. (17) Amara, A., Gall, S. L., Schwartz, O., Salamero, J., Montes, M., Loetscher, P., Baggiolini, M., Virelizier, J. L., and Arenzana-Seisdedos, F. (1997) HIV coreceptor downregulation as antiviral principle: SDF1alpha-dependent internalization of the chemokine receptor CXCR4 contributes to inhibition of HIV replication. J. Exp. Med. 186, 139− 146. (18) Altenburg, J. D., Jin, Q., Alkhatib, B., and Alkhatib, G.. (2010) The potent anti-HIV activity of CXCL12γ correlates with efficient CXCR4 binding and internalization. J. Virol. 84, 2563−2572. (19) Tamamura, H., Xu, Y., Hattori, T., Zhang, X., Arakaki, R., Kanbara., K., Omagari, A., Otaka, A., Ibuka, T., Yamamoto, N., Nakashima, H., and Fujii, N. (1998) A low-molecular-weight inhibitor against the chemokine receptor CXCR4: a strong anti-HIV peptide T140. Biochem. Biophys. Res. Commun. 253, 877−882. (20) Gerlach, L. O., Skerlj, R. T., Bridger, G. J., and Schwartz, T. W. (2001) Molecular interactions of cyclam and bicyclam non-peptide antagonists with the CXCR4 chemokine receptor. J. Biol. Chem. 276, 14153−14160. (21) Oishi, S., Masuda, R., Evans, B., Ueda, S., Goto, Y., Ohno, H., Hirasawa, A., Tsujimoto, G., Wang, Z., Peiper, S. C., Naito, T., Kodama, E., Matsuoka, M., and Fujii, N. (2008) Synthesis and application of fluorescein- and biotin-labeled molecular probes for the chemokine receptor CXCR4. ChemBioChem 9, 1154−1158. (22) Masuda, R., Oishi, S., Ohno, H., Kimura, H., Saji, H., and Fujii, N. (2011) Concise site-specific synthesis of DTPA-peptide conjugates: application to imaging probes for the chemokine receptor CXCR4. Bioorg. Med. Chem. 19, 3216−3220. (23) Zhang, W. B., Navenot, J. M., Haribabu, B., Tamamura, H., Hiramatu, K., Omagari, A., Pei, G., Manfredi, J. P., Fujii, N., Broach, J. R., and Peiper, S. C. (2002) A point mutation that confers constitutive activity to CXCR4 reveals that T140 is an inverse agonist and that AMD3100 and ALX40−4C are weak partial agonists. J. Biol. Chem. 277, 24515−24521. (24) Wu, B., Chien, E. Y., Mol, C. D., Fenalti, G., Liu, W., Katritch, V., Abagyan, R., Brooun, A., Wells, P., Bi, F. C., Hamel, D. J., Kuhn, P., Handel, T. M., Cherezov, V., and Stevens, R. C. (2010) Structures of the CXCR4 chemokine GPCR with small-molecule and cyclic peptide antagonists. Science 330, 1066−1071. (25) Crump, M. P., Gong, J. H., Loetscher, P., Rajarathnam, K., Amara, A., Arenzana-Seisdedos, F., Virelizier, J. L., Baggiolini, M., Sykes, B. D., and Clark-Lewis, I. (1997) Solution structure and basis for functional activity of stromal cell-derived factor-1; dissociation of CXCR4 activation from binding and inhibition of HIV-1. EMBO J. 16, 6996−7007. (26) Trent, J. O., Wang, Z. X., Murray, J. L., Shao, W., Tamamura, H., Fujii, N., and Peiper, S. C. (2003) Lipid bilayer simulations of CXCR4 with inverse agonists and weak partial agonists. J. Biol. Chem. 278, 47136−47144. (27) Doranz, B. J., Grovit-Ferbas, K., Sharron, M. P., Mao, S. H., Goetz, M. B., Daar, E. S., Doms, R. W., and O’Brien, W. A. (1997) A small-molecule inhibitor directed against the chemokine receptor CXCR4 prevents its use as an HIV-1 coreceptor. J. Exp. Med. 186, 1395−1400. (28) Futaki, S., Nakase, I., Tadokoro, A., Takeuchi, T., and Jones, A. T. (2007) Arginine-rich peptides and their internalization mechanisms. Biochem. Soc. Trans. 35, 784−787.

AUTHOR INFORMATION

Corresponding Author

*Tel: +81-75-753-4551, Fax: +81-75-753-4570, E-mail: soishi@ pharm.kyoto-u.ac.jp (S.O.), [email protected] (N.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Prof. Shiroh Futaki and Dr. Ikuhiko Nakase (Institute for Chemical Research, Kyoto University) for excellent technical advices. This work was supported by Grants-in-Aid for Scientific Research and Targeted Proteins Research Program from the Ministry of Education, Culture, Sports, Science, and Technology of Japan; and by a grant for Promotion of AIDS Research from the Ministry of Health and Welfare of Japan. R.M. is grateful for Research Fellowships from the JSPS for Young Scientists.

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